 |
Introduction |  |
This booklet was put together to
familiarize the general reader with the terminology of suspension
bridge components and to help the designer, builder or user of
a small suspension bridge. Its use should enable him to make up
preliminary calculations for determining the cable size as well
as the various quantities of material required. Then, a comparative
estimate can be made between the suspension bridge and any other
type that may also be under consideration for a particular location.
It is rather interesting to note
that, in spite of the relative simplicity of design and erection
of a suspension bridge, there are a number of cases where other
types have been used, even though the suspension type might have
been more economical. We think that this is because many engineers
have been of the opinion that the cable analysis might be difficult
and complicated as to its solution. However, the simple formulae
used in the catalog should dispel this thought.
All we ask is that the imaginative
engineer try the suspension type the next time he contemplates
building a bridge.
It is also our hope that the
experienced designer of suspension bridges may find this booklet
of use as a source of shortcut methods for arriving at his
first approximations, and for the solution of many cable erection
problems.
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Terminology | |
Suspension Bridge Data
Galvanized Bridge Wire for Parallel Wire
Bridge Cables. Recommended diameter .196 inch.
Galvanized Bridge Strand--consists of
several bridge wires, of various diameters
twisted together.
Galvanized Bridge Rope--consists of six
strands twisted around a strand core.
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Types of Suspension Bridge Cables | |
1. Parallel Wire Cables--This type of cable is made
up of a large number of individual wires which are parallel to
one another. Neither the cables nor the wires are twisted in any
manner. The wire i6 shipped to the site of the bridge on reels
and the individual wires are installed or' "spun" on
the bridge and later compacted together to form a round crosssection.
Cables of this type are used on monumental structures, such as
the Golden Gate Bridge and the George Washington Bridge.
Parallel Wire Cable
2. Parallel Strand Cables, Closed
Construction--These consist
of several prefabricated Galvanized Bridge Strands, all laid parallel
and in contact with one another. Wood or aluminum fillers are
used to bring the cable to a circular cross-section, after which
the whole cable is wrapped with wire for protection. The cable
may contain 7, 19 37, 61, 91 or 127 strands.
Detail of Main Cable and Cable Band. The wrapping
wire is omitted at the right for clarity. Note the
closed construction and aluminum fillers.
3. Parallel Strand Cables, Open Construction--This type of
cable consists of several prefabricated Galvanized bridge Strands
which are all laid
parallel to one another and not in contact. The strands are usually
arranged in the form of a rectangle and the cable may contain
2, 4, 6, 9, 12, 16, 20, 24 or 30 strands.
4. Parallel Rope Cables, Open Construction--These are the
same as Parallel Strand Cables except that Galvanized Bridge Rope
is used in place of Bridge
Strand.
Close-up view of Main Cable, Cable Bend
and Suspender. Note the open construction.
5. Single Rope or Single Strand Cables--These are used
for small structures.
Cable with Clip Type Cable Band
and Suspender.
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Prestretching Galvanized Bridge Strands and Galvanized bridge Ropes | |
For many years the main cables of
most suspension bridges, large and small, consisted of parallel
wires installed individually at the site of the bridge. On small
bridges this proved to be an expensive procedure and consequently
placed the suspensiontype bridge at an economic disadvantage
for the shorter span crossings.
The use of prefabricated strands
for these cables, although much less costly in erection, was restricted
because the elastic properties of the strands could not he predicted
and were not stabilized until the bridge had been in service for
some time.
In 1928, however, Roebling developed
the process known as "Prestretching," whereby the elastic
properties of a prefabricated strand or rope can he definitely
established. This operation consists of subjecting the member
to a tension above its working tension and holding it there until
the desired results have been obtained. Since the time this practice
was initiated, it has been possible for the designer to depend
on a length of prefabricated Galvanized Bridge Strand or Bridge
Rope with the same confidence that he places in the length of
a structural steel member. The immediate result was to make the
suspension bridge economically advantageous for much shorter span
lengths. Prestretched Galvanized Bridge Strand can he depended
on to have a minimum modulus of elasticity of 24,000,000 lbs.
per sq. in. and the minimum modulus of Prestretched Galvanized
Bridge Rope is 20,000,000 lbs. per sq. in.
The prestretching procedure also
makes it possible to measure the prefabricated members to exact
lengths in the shop under their working tension. Furthermore,
the location of the centerline of the main tower saddle can he
established and marked on each main cable. The location of each
cable band can also be established and marked on one member for
each main cable.
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Notes on Suspension Bridge Design | |
The suspension bridge is inherently
a flexible structure and in the majority of cases some form of
stiffening must he incorporated in the design. On highway or walkway
bridges, stiffness may he obtained with a stiffening truss or
by a properly designed diagonal cable system. In statically loaded
bridges, such as pipe lines or belt conveyors, the stiffening
truss may be omitted.
The effect of heavy transverse winds
on a suspension bridge necessitates the incorporation of a properly
designed wind bracing system. This wind bracing system can be
incorporated in the floor system design or the necessary stiffness
may he obtained more economically in some cases by the use of
a wind cable system.
The choice of main cable size can
be made after the total dead and live loads have been determined.
A quick, approximate estimate of the cable size can be made as follows
(see figure 1) :
The proportions of the stiffening
truss may be approximated with sufficient accuracy for a rough
estimate by designing a weightless truss which will support the
uniform live load over a simple span equal to 40'.S of the main
span of the bridge. The wind bracing system may also be approximated
by using conventional design methods.
The design of the wind cable system
is similar to the design of the main cable. However, the design
of a diagonal cable stiffening system requires a different analysis
and Roebling's Bridge Division is available to assist in the design.
Inquiries in connection with the design of specific projects are
invited.
For the final design of the bridge
an accurate analysis should be made and the complete coverage
of this subject would require a booklength discussion. The
reader who wishes to acquaint himself with this branch of engineering,
however, is referred to the published literature. Many such sources
of information are listed in the following publication "A
History of Suspension Bridges in Bibliographical Form" by
A. A. Jakkula, a Bulletin of the Agricultural & Mechanical
College of Texas.
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Notes on Suspension Bridge Erection Calculations | |
An important item to be determined
by the computer of suspension bridge calculations is the free
cable sag-the elevation at which the cables must be set when hanging
under their own weight only, to make certain that the fullyloaded
bridge will come k~ rest at the right elevation. Obviously, the
cable has one length under dead load tension and a shorter length
under free cable tension due to elastic contraction. However,
it always has the same unstressed length from anchor to anchor.
If this one constant, common characteristic is kept in mind, the
problem of finding the free cable sag is reduced to a simple form
of applied mechanics.
The unstressed length is found for
each span of the bridge from the dead load condition. By trial
and error a free cable HF is found which is equal
for all spans and yields the proper
unstressed length in each span. The free cable curves may be computed
accurately by the use of catenary formulas, such as shown in figure
1, or approximated by the use of the parabolic formulas (equations
114) listed under "Approximate Formulas
for
Determining Cable and Suspender lengths and Cable Tensions."
A rapid approximation of free cable characteristics may be made as follows:
due to dead load H in each span.
for all spans from anchor to anchor.
for this HF. In this table there will be a column for
each span and each trial will involve the use of a group of
columns representing all the spans from anchor to anchor.
equal to that found in step 3, it is the correct value for HF.
On larger bridges it is also necessary to make computations for sag
temperature charts. These charts define the line of sight necessary
to estab-lish the proper sag of the free hanging strand in any span
for any temperature and tower position.
In recent years an alternative method of manufacture and erection
procedure for the prefabric-ated main cable strands has been evolved
which eliminates the time-consuming and voluminous computations
necessary to prepare the sag temp-erature charts. Under this new method
the first strands are-erected with normal shims at the an-chorages and
with the centerline of main tower saddle marks placed exactly on the
prescribed position. Thereafter additional strands are simply adjusted
to the first strand. This method requires particular care to be taken
to check the exact location of the main towers and anchorages. It also
requires the cable manufacturer to take special precautions in the
measuring and handling of the main cable strands. It is suggested that
this method of erection be discussed with Roebling before being
adopted.
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Charts, Tables, and Technical Data | |
| Standard Galvanized Steel Bridge Rope | 67KB JPEG |
| Standard Galvanized Steel Bridge Strand | 56KB JPEG |
| Approximate Formulas for Determining Cable and Suspender Lengths and Cable Tensions | 29KB JPEG 50KB JPEG |
| Main Span Suspender Lengths | 20KB JPEG |
| Side Span Suspender Lengths | 23KB JPEG |
| Catenary Formulas (Figure 1) | 29KB JPEG |
| Approximate Computation of Free Cable H | 45KB JPEG |
| Chart 1A | 108KB JPEG |
| Chart 1B | 76KB JPEG |
| Chart 1C | 120KB JPEG |
